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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




The second case is that detailed by a no less unexceptionable authority
than Réaumur, in his "Art de faire éclore les Poulets." A Maltese couple,
named Kelleia, whose hands and feet were constructed upon the ordinary
human model, had born to them a son, Gratio, who possessed six perfectly
movable fingers on each hand, and six toes, not quite so well formed, on
each foot. No cause could be assigned for the appearance of this unusual
variety of the human species.

Two circumstances are well worthy of remark in both these cases. In each,
the variety appears to have arisen in full force, and, as it were, _per
saltum_; a wide and definite difference appearing, at once, between the
Ancon ram and the ordinary sheep; between the six-fingered and six-toed
Gratio Kelleia and ordinary men. In neither case is it possible to point
out any obvious reason for the appearance of the variety. Doubtless there
were determining causes for these as for all other phenomena; but they do
not appear, and we can be tolerably certain that what are ordinarily
understood as changes in physical conditions, as in climate, in food, or
the like, did not take place and had nothing to do with the matter. It was
no case of what is commonly called adaptation to circumstances; but, to use
a conveniently erroneous phrase, the variations arose spontaneously. The
fruitless search after final causes leads their pursuers a long way; but
even those hardy teleologists, who are ready to break through all the laws
of physics in chase of their favourite will-o'-the-wisp, may be puzzled to
discover what purpose could be attained by the stunted legs of Seth
Wright's ram or the hexadactyle members of Gratio Kelleia.

Varieties then arise we know not why; and it is more than probable that the
majority of varieties have arisen in this "spontaneous" manner, though we
are, of course, far from denying that they may be traced, in some cases, to
distinct external influences; which are assuredly competent to alter the
character of the tegumentary covering, to change colour, to increase or
diminish the size of muscles, to modify constitution, and, among plants, to
give rise to the metamorphosis of stamens into petals, and so forth. But
however they may have arisen, what especially interests us at present is,
to remark that, once in existence, many varieties obey the fundamental law
of reproduction that like tends to produce like; and their offspring
exemplify it by tending to exhibit the same deviation from the parental
stock as themselves. Indeed, there seems to be, in many instances, a
prepotent influence about a newly-arisen variety which gives it what one
may call an unfair advantage over the normal descendants from the same
stock. This is strikingly exemplified by the case of Gratio Kelleia, who
married a woman with the ordinary pentadactyle extremities, and had by her
four children, Salvator, George, André, and Marie. Of these children
Salvator, the eldest boy, had six fingers and six toes, like his father;
the second and third, also boys, had five fingers and five toes, like their
mother, though the hands and feet of George were slightly deformed. The
last, a girl, had five fingers and five toes, but the thumbs were slightly
deformed. The variety thus reproduced itself purely in the eldest, while
the normal type reproduced itself purely in the third, and almost purely in
the second and last: so that it would seem, at first, as if the normal type
were more powerful than the variety. But all these children grew up and
intermarried with normal wives and husband, and then, note what took place:
Salvator had four children, three of whom exhibited the hexadactyle members
of their grandfather and father, while the youngest had the pentadactyle
limbs of the mother and grandmother; so that here, notwithstanding a double
pentadactyle dilution of the blood, the hexadactyle variety had the best of
it. The same pre-potency of the variety was still more markedly exemplified
in the progeny of two of the other children, Marie and George. Marie (whose
thumbs only were deformed) gave birth to a boy with six toes, and three
other normally formed children; but George, who was not quite so pure a
pentadactyle, begot, first, two girls, each of whom had six fingers and
toes; then a girl with six fingers on each hand and six toes on the right
foot, but only five toes on the left; and lastly, a boy with only five
fingers and toes. In these instances, therefore, the variety, as it were,
leaped over one generation to reproduce itself in full force in the next.
Finally, the purely pentadactyle André was the father of many children, not
one of whom departed from the normal parental type.

If a variation which approaches the nature of a monstrosity can strive thus
forcibly to reproduce itself, it is not wonderful that less aberrant
modifications should tend to be preserved even more strongly; and the
history of the Ancon sheep is, in this respect, particularly instructive.
With the "'cuteness" characteristic of their nation, the neighbours of the
Massachusetts farmer imagined it would be an excellent thing if all his
sheep were imbued with the stay-at-home tendencies enforced by Nature upon
the newly-arrived ram; and they advised Wright to kill the old patriarch of
his fold, and install the Ancon ram in his place. The result justified
their sagacious anticipations, and coincided very nearly with what occurred
to the progeny of Gratio Kelleia. The young lambs were almost always either
pure Ancons, or pure ordinary sheep.[Footnote: Colonel Humphreys'
statements are exceedingly explicit on this point:--. "When an Ancon ewe is
impregnated by a common ram, the increase resembles wholly either the ewe
or the ram. The increase of the common ewe impregnated by an Ancon ram
follows entirely the one or the other, without blending any of the
distinguishing and essential peculiarities of both. Frequent instances have
happened where common ewes have had twins by Ancon rams, when one exhibited
the complete marks and features of the ewe, the other of the ram. The
contrast has been rendered singularly striking, when one short-legged and
one long-legged lamb, produced at a birth, have been seen sucking the dam
at the same time."--_Philosophical Transactions_, 1813, Ft. I. pp. 89,
90.] But when sufficient Ancon sheep were obtained to interbreed with one
another, it was found that the offspring was always pure Ancon. Colonel
Humphreys, in fact, states that he was acquainted with only "one
questionable case of a contrary nature." Here, then, is a remarkable and
well-established instance, not only of a very distinct race being
established _per saltum_, but of that race breeding "true" at once,
and showing no mixed forms, even when crossed with another breed.

By taking care to select Ancons of both sexes, for breeding from, it thus
became easy to establish an extremely well-marked race; so peculiar that,
even when herded with other sheep, it was noted that the Ancons kept
together. And there is every reason to believe that the existence of this
breed might have been indefinitely protracted; but the introduction of the
Merino sheep, which were not only very superior to the Ancons in wool and
meat, but quite as quiet and orderly, led to the complete neglect of the
new breed, so that, in 1813, Colonel Humphreys found it difficult to obtain
the specimen, the skeleton of which was presented to Sir Joseph Banks. We
believe that, for many years, no remnant of it has existed in the United
States.

Gratio Kelleia was not the progenitor of a race of six-fingered men, as
Seth Wright's ram became a nation of Ancon sheep, though the tendency of
the variety to perpetuate itself appears to have been fully as strong in
the one case as in the other. And the reason of the difference is not far
to seek. Seth Wright took care not to weaken the Ancon blood by matching
his Ancon ewes with any but males of the same variety, while Gratio
Kelleia's sons were too far removed from the patriarchal times to
intermarry with their sisters; and his grand-children seem not to have been
attracted by their six-fingered cousins. In other words, in the one example
a race was produced, because, for several generations, care was taken to
_select_ both parents of the breeding stock from animals exhibiting a
tendency to vary in the same direction; while, in the other, no race was
evolved, because no such selection was exercised. A race is a propagated
variety; and as, by the laws of reproduction, offspring tend to assume the
parental forms, they will be more likely to propagate a variation exhibited
by both parents than that possessed by only one.

There is no organ of the body of an animal which may not, and does not,
occasionally, vary more or less from the normal type; and there is no
variation which may not be transmitted and which, if selectively
transmitted, may not become the foundation of a race. This great truth,
sometimes forgotten by philosophers, has long been familiar to practical
agriculturists and breeders; and upon it rest all the methods of improving
the breeds of domestic animals, which, for the last century, have been
followed with so much success in England. Colour, form, size, texture of
hair or wool, proportions of various parts, strength or weakness of
constitution, tendency to fatten or to remain lean, to give much or little
milk, speed, strength, temper, intelligence, special instincts; there is
not one of these characters the transmission of which is not an every-day
occurrence within the experience of cattle-breeders, stock-farmers,
horse-dealers, and dog and poultry fanciers. Nay, it is only the other day
that an eminent physiologist, Dr. Brown-Séquard, communicated to the Royal
Society his discovery that epilepsy, artificially produced in guinea-pigs,
by a means which he has discovered, is transmitted to their offspring.
[Footnote: Compare Weismann's _Essays Upon Heredity_, p. 310, _et
seq_. 1893.]

But a race, once produced, is no more a fixed and immutable entity than the
stock whence it sprang; variations arise among its members, and as these
variations are transmitted like any others, new races may be developed out
of the pre-existing one _ad infinitum_, or, at least, within any limit
at present determined. Given sufficient time and sufficiently careful
selection, and the multitude of races which may arise from a common stock
is as astonishing as are the extreme structural differences which they may
present. A remarkable example of this is to be found in the rock-pigeon,
which Mr. Darwin has, in our opinion, satisfactorily demonstrated to be the
progenitor of all our domestic pigeons, of which there are certainly more
than a hundred well-marked races. The most noteworthy of these races are,
the four great stocks known to the "fancy" as tumblers, pouters, carriers,
and fantails; birds which not only differ most singularly in size, colour,
and habits, but in the form of the beak and of the skull; in the
proportions of the beak to the skull; in the number of tail-feathers; in
the absolute and relative size of the feet; in the presence or absence of
the uropygial gland; in the number of vertebræ in the back; in short, in
precisely those characters in which the genera and species of birds differ
from one another.

And it is most remarkable and instructive to observe, that none of these
races can be shown to have been originated by the action of changes in what
are commonly called external circumstances, upon the wild rock-pigeon. On
the contrary, from time immemorial pigeon-fanciers have had essentially
similar methods of treating their pets, which have been housed, fed,
protected and cared for in much the same way in all pigeonries. In fact,
there is no case better adapted than that of the pigeons to refute the
doctrine which one sees put forth on high authority, that "no other
characters than those founded on the development of bone for the attachment
of muscles" are capable of variation. In precise contradiction of this
hasty assertion, Mr. Darwin's researches prove that the skeleton of the
wings in domestic pigeons has hardly varied at all from that of the wild
type; while, on the other hand, it is in exactly those respects, such as
the relative length of the beak and skull, the number of the vertebrae, and
the number of the tail-feathers, in which muscular exertion can have no
important influence, that the utmost amount of variation has taken place.

We have said that the following out of the properties exhibited by
physiological species would lead us into difficulties, and at this point
they begin to be obvious; for if, as the result of spontaneous variation
and of selective breeding, the progeny of a common stock may become
separated into groups distinguished from one another by constant, not
sexual, morphological characters, it is clear that the physiological
definition of species is likely to clash with the morphological definition.
No one would hesitate to describe the pouter and the tumbler as distinct
species, if they were found fossil, or if their skins and skeletons were
imported, as those of exotic wild birds commonly are--and without doubt, if
considered alone, they are good and distinct morphological species. On the
other hand, they are not physiological species, for they are descended from
a common stock, the rock-pigeon.

Under these circumstances, as it is admitted on all sides that races occur
in Nature, how are we to know whether any apparently distinct animals are
really of different physiological species, or not, seeing that the amount
of morphological difference is no safe guide? Is there any test of a
physiological species? The usual answer of physiologists is in the
affirmative. It is said that such a test is to be found in the phænomena of
hybridisation--in the results of crossing races, as compared with the
results of crossing species.

So far as the evidence goes at present, individuals, of what are certainly
known to be mere races produced by selection, however distinct they may
appear to be, not only breed freely together, but the offspring of such
crossed races are perfectly fertile with one another. Thus, the spaniel and
the greyhound, the dray-horse and the Arab, the pouter and the tumbler,
breed together with perfect freedom, and their mongrels, if matched with
other mongrels of the same kind, are equally fertile.

On the other hand, there can be no doubt that the individuals of many
natural species are either absolutely infertile if crossed with individuals
of other species, or, if they give rise to hybrid offspring, the hybrids so
produced are infertile when paired together. The horse and the ass, for
instance, if so crossed, give rise to the mule, and there is no certain
evidence of offspring ever having been produced by a male and female mule.
The unions of the rock-pigeon and the ring-pigeon appear to be equally
barren of result. Here, then, says the physiologist, we have a means of
distinguishing any two true species from any two varieties. If a male and a
female, selected from each group, produce offspring, and that offspring is
fertile with others produced in the same way, the groups are races and not
species. If, on the other hand, no result ensues, or if the offspring are
infertile with others produced in the same way, they are true physiological
species. The test would be an admirable one, if, in the first place, it
were always practicable to apply it, and if, in the second, it always
yielded results susceptible of a definite interpretation. Unfortunately, in
the great majority of cases, this touchstone for species is wholly
inapplicable.

The constitution of many wild animals is so altered by confinement that
they will not breed even with their own females, so that the negative
results obtained from crosses are of no value; and the antipathy of wild
animals of different species for one another, or even of wild and tame
members of the same species, is ordinarily so great, that it is hopeless to
look for such unions in Nature. The hermaphrodism of most plants, the
difficulty in the way of insuring the absence of their own or the proper
working of other pollen, are obstacles of no less magnitude in applying the
test to them. And, in both animals and plants, is super-added the further
difficulty, that experiments must be continued over a long time for the
purpose of ascertaining the fertility of the mongrel or hybrid progeny, as
well as of the first crosses from which they spring.

Not only do these great practical difficulties lie in the way of applying
the hybridisation test, but even when this oracle can be questioned, its
replies are sometimes as doubtful as those of Delphi. For example, cases
are cited by Mr. Darwin, of plants which are more fertile with the pollen
of another species than with their own; and there are others, such as
certain _Fuci,_ the male element of which will fertilise the ovule of
a plant of distinct species, while the males of the latter species are
ineffective with the females of the first. So that, in the last-named
instance, a physiologist, who should cross the two species in one way,
would decide that they were true species; while another, who should cross
them in the reverse way, would, with equal justice, according to the rule,
pronounce them to be mere races. Several plants, which there is great
reason to believe are mere varieties, are almost sterile when crossed;
while both animals and plants, which have always been regarded by
naturalists as of distinct species, turn out, when the test is applied, to
be perfectly fertile. Again, the sterility or fertility of crosses seems to
bear no relation to the structural resemblances or differences of the
members of any two groups.

Mr. Darwin has discussed this question with singular ability and
circumspection, and his conclusions are summed up as follows, at page 276
of his work:--

"First crosses between forms sufficiently distinct to be ranked as species,
and their hybrids, are very generally, but not universally, sterile. The
sterility is of all degrees, and is often so slight that the two most
careful experimentalists who have ever lived have come to diametrically
opposite conclusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and is eminently
susceptible of favourable and unfavourable conditions. The degree of
sterility does not strictly follow systematic affinity, but is governed by
several curious and complex laws. It is generally different and sometimes
widely different, in reciprocal crosses between the same two species. It is
not always equal in degree in a first cross, and in the hybrid produced
from this cross.

"In the same manner as in grafting trees, the capacity of one species or
variety to take on another is incidental on generally unknown differences
in their vegetative systems; so in crossing, the greater or less facility
of one species to unite with another is incidental on unknown differences
in their reproductive systems. There is no more reason to think that
species have been specially endowed with various degrees of sterility to
prevent them crossing and breeding in Nature, than to think that trees have
been specially endowed with various and somewhat analogous degrees of
difficulty in being grafted together, in order to prevent them becoming
inarched in our forests.

"The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances; in
some cases largely on the early death of the embryo. The sterility of
hybrids which have their reproductive systems imperfect, and which have had
this system and their whole organisation disturbed by being compounded of
two distinct species, seems closely allied to that sterility which so
frequently affects pure species when their natural conditions of life have
been disturbed. This view is supported by a parallelism of another kind:
namely, that the crossing of forms, only slightly different, is favourable
to the vigour and fertility of the offspring; and that slight changes in
the conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree of
difficulty in uniting two species, and the degree of sterility of their
hybrid offspring, should generally correspond, though due to distinct
causes; for both depend on the amount of difference of some kind between
the species which are crossed. Nor is it surprising that the facility of
effecting a first cross, the fertility of hybrids produced from it, and the
capacity of being grafted together--though this latter capacity evidently
depends on widely different circumstances--should all run to a certain
extent parallel with the systematic affinity of the forms which are
subjected to experiment; for systematic affinity attempts to express all
kinds of resemblance between all species.

"First crosses between forms known to be varieties, or sufficiently alike
to be considered as varieties, and their mongrel offspring, are very
generally, but not quite universally, fertile. Nor is this nearly general
and perfect fertility surprising, when we remember how liable we are to
argue in a circle with respect to varieties in a state of Nature; and when
we remember that the greater number of varieties have been produced under
domestication by the selection of mere external differences, and not of
differences in the reproductive system. In all other respects, excluding
fertility, there is a close general resemblance between hybrids and
mongrels."--Pp. 276-8.

We fully agree with the general tenor of this weighty passage; but forcible
as are these arguments, and little as the value of fertility or infertility
as a test of species may be, it must not be forgotten that the really
important fact, so far as the inquiry into the origin of species goes, is,
that there are such things in Nature as groups of animals and of plants,
the members of which are incapable of fertile union with those of other
groups; and that there are such things as hybrids, which are absolutely
sterile when crossed with other hybrids. For, if such phænomena as these
were exhibited by only two of those assemblages of living objects, to which
the name of species (whether it be used in its physiological or in its
morphological sense) is given, it would have to be accounted for by any
theory of the origin of species, and every theory which could not account
for it would be, so far, imperfect.

Up to this point, we have been dealing with matters of fact, and the
statements which we have laid before the reader would, to the best of our
knowledge, be admitted to contain a fair exposition of what is at present
known respecting the essential properties of species, by all who have
studied the question. And whatever may be his theoretical views, no
naturalist will probably be disposed to demur to the following summary of
that exposition:--

Living beings, whether animals or plants, are divisible into multitudes of
distinctly definable kinds, which are morphological species. They are also
divisible into groups of individuals, which breed freely together, tending
to reproduce their like, and are physiological species. Normally resembling
their parents, the offspring of members of these species are still liable
to vary; and the variation may be perpetuated by selection, as a race,
which race, in many cases, presents all the characteristics of a
morphological species. But it is not as yet proved that a race ever
exhibits, when crossed with another race of the same species, those
phænomena of hybridisation which are exhibited by many species when crossed
with other species. On the other hand, not only is it not proved that all
species give rise to hybrids infertile _inter se_, but there is much
reason to believe that, in crossing, species exhibit every gradation from
perfect sterility to perfect fertility.

Such are the most essential characteristics of species. Even were man not
one of them--a member of the same system and subject to the same laws--the
question of their origin, their causal connexion, that is, with the other
phænomena of the universe, must have attracted his attention, as soon as
his intelligence had raised itself above the level of his daily wants.

Indeed history relates that such was the case, and has embalmed for us the
speculations upon the origin of living beings, which were among the
earliest products of the dawning intellectual activity of man. In those
early days positive knowledge was not to be had, but the craving after it
needed, at all hazards, to be satisfied, and according to the country, or
the turn of thought, of the speculator, the suggestion that all living
things arose from the mud of the Nile, from a primeval egg, or from some
more anthropomorphic agency, afforded a sufficient resting-place for his
curiosity. The myths of Paganism are as dead as Osiris or Zeus, and the man
who should revive them, in opposition to the knowledge of our time, would
be justly laughed to scorn; but the coeval imaginations current among the
rude inhabitants of Palestine, recorded by writers whose very name and age
are admitted by every scholar to be unknown, have unfortunately not yet
shared their fate, but, even at this day, are regarded by nine-tenths of
the civilised world as the authoritative standard of fact and the criterion
of the justice of scientific conclusions, in all that relates to the origin
of things, and, among them, of species. In this nineteenth century, as at
the dawn of modern physical science, the cosmogony of the semi-barbarous
Hebrew is the incubus of the philosopher and the opprobrium of the
orthodox. Who shall number the patient and earnest seekers after truth,
from the days of Galileo until now, whose lives have been embittered and
their good name blasted by the mistaken zeal of Bibliolaters? Who shall
count the host of weaker men whose sense of truth has been destroyed in the
effort to harmonise impossibilities--whose life has been wasted in the
attempt to force the generous new wine of Science into the old bottles of
Judaism, compelled by the outcry of the same strong party?

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